The valence electron structure and cohesive energy of α-Al, AlB₂ and (Al-Si)B₂ crystals were calculated using the empirical electron theory (EET) of solids and molecules. The calculated results indicate that Al-Al atomic layer on outermost surface of AlB₂ is relatively unstable and the cohesive energy of both α-Al and AlB₂ decrease with increase of Si content in Al-Si melt. According to the calculated results, a novel atomic mechanism of α-Al heterogeneously nucleating on AlB₂ in Al-Si alloy is explored. After adding additional Si, a certain amount of Si atoms enter into AlB₂, which results in formation of a stable Al-Si binary atomic structure layer on AlB₂ surface and finally improves the stability of AlB₂. This two-dimensional Al-Si atomic layer plays an important transition role in the subsequent heterogeneous nucleation process, which is responsible for the atomic mechanism of nucleation of α-Al attached to AlB₂.